I. Field of the Invention
The present invention relates generally to mobile radio communication systems. More particularly, the present invention relates to mobile radio units that can receive and demodulate signals at different frequency bands that have been modulated in accordance with multiple modulation formats. Even more particularly, the present invention relates to a novel and improved receiver circuit that downconverts such signals at different frequency bands to a common frequency band using a minimal number of hardware components.
II. Description of the Prior Art
Currently, mobile radio telephones typically support communications in a number of different modes corresponding to different modulation formats. For example, there are so-called xe2x80x9cdual-modexe2x80x9d mobile radio telephones that support communications using both analog and code division multiple access (CMDA) signals. As the number of modes that the phone supports increases, the circuitry required in the phone for downconverting and sampling the input signals typically increases in complexity. This occurs because, in multi-mode phones, the input signals are received at different frequency bands depending on the operating mode, and downconverting and sampling the signals from each of the frequency bands typically requires separate circuitry for each band. It would therefore be desirable to have a receiver design that could be used in multi-mode phones for downconverting and sampling the input signals which used common hardware for the downconversion and sampling operations, thereby minimizing the hardware components required for operation of a multi-mode phone.
There present invention is directed to a receiver that downconverts input signals modulated using first, second, third and fourth modulation formats to a common intermediate frequency range. The first and second modulation formats are transmitted to the receiver in a first frequency range, the third modulation format is transmitted to the receiver in a second frequency range, and the fourth modulation format is transmitted to the receiver in a third frequency range. The input signals are provided to first, second and third band selection filters that respectively select first, second and third frequency ranges. A first downconverter is coupled to an output of the first band selection filter, and downconverts signals from the first frequency range to the common intermediate frequency range. A second downconverter is selectively coupled by a switch to either an output of the second band selection filter or an output of the third band selection filter, and downconverts signals from either the second frequency range or the third frequency range to the common intermediate frequency range. The second downconverter has an input coupled to a frequency doubling circuit. Switching circuitry selectively couples one of either a first oscillating signal from a voltage controlled oscillator (VCO) having a VCO frequency range or a second oscillating signal at a second frequency that is outside the VCO frequency range to an input of the first downconverter and an input of the frequency doubling circuit
In a preferred embodiment, the VCO is responsive to control circuitry and selectively generates the first oscillating signal at a channel frequency associated with the first modulation format, and the first downconverter mixes the output of the first band selection filter with the first oscillating signal at the channel frequency associated with the first modulation format in order to downconvert signals modulated in accordance with the first modulation format from the first frequency range to the common intermediate frequency range. The VCO further selectively generates the first oscillating signal at a channel frequency associated with the second modulation format, and the first downconverter mixes the output of the first band selection filter with the first oscillating signal at the channel frequency associated with the second modulation format in order to downconvert signals modulated in accordance with the second modulation format from the first frequency range to the common intermediate frequency range. In addition, in the preferred embodiment, the VCO further selectively generates the first oscillating signal at a channel frequency associated with the third modulation format, and the first oscillating signal at the channel frequency associated with the third modulation format is provided to the frequency doubling circuit. The second downconverter then mixes the output of the second band selection filter with an output of the frequency doubling circuit when the first oscillating signal at a channel frequency associated with the third modulation format is provided to the frequency doubling circuit in order to downconvert signals modulated in accordance with the third modulation format from the second frequency range to the common intermediate frequency range.
The receiver of the present invention also preferably includes a mixer that forms the second oscillating signal at the second frequency by offsetting the frequency of the first oscillating signal. The second oscillating signal is selectively provided to the frequency doubling circuit, and the second downconverter mixes the output of the third band selection filter with the output of the frequency doubling circuit when the second oscillating signal is provided to the frequency doubling circuit in order to downconvert signals modulated in accordance with the fourth modulation format from the third frequency range to the common intermediate frequency range.
In accordance with a further aspect, the receiver of the present invention includes one or more channel selection filters that are coupled to outputs of the first and second downconverters. The channel selection filter(s) function to filter the downconverted signals output by the first and second downconverters. A sampling circuit is coupled to an output of the channel selection filter(s). The sampling circuit selectively samples the downconverted signals with a sampling clock that alternatively samples signals at either a first or second sampling rate. The sampling clock is coupled to a third oscillating signal at a third frequency, and the third oscillating signal is an input to the mixer that forms the second oscillating signal at the second frequency. The first sampling rate provided by the sampling clock is equal to the third frequency divided by x, and the second sampling rate provided by the sampling clock is equal to the third frequency divided by y, wherein x and y are integers and, in one example, are equal to 3 and 15. This aspect invention minimizes excess hardware in the receiver, since the third oscillating signal at the third frequency is used both in the frequency downconversion process as well as in the sampling process. In one embodiment, the sampling circuit samples downconverted signals modulated in accordance with either the second, third or fourth modulation formats in accordance with the first sampling rate, and the sampling circuit samples downconverted signals modulated in accordance with the first modulation format in accordance with the second sampling rate.
In one embodiment, only a single channel selection filter is coupled to outputs of the first and second downconverters, and the single channel selection filter operates to filter downconverted signals modulated in accordance with the first and second modulation formats output by the first downconverter, and the single channel selection filter further filters downconverted signals modulated in accordance with the third and fourth modulation formats output by the second downconverter.
In a further embodiment, first and second channel selection filters are coupled to outputs of the first and second downconverters. The first channel selection filter operates to filter downconverted signals modulated in accordance with the first modulation format output by the first downconverter, the second channel selection filter operates to filter downconverted signals modulated in accordance with the second modulation format output by the first downconverter, the second channel filter also filters downconverted signals modulated in accordance with the third modulation format output by the second downconverter, and the second channel selection filter further filters downconverted signals modulated in accordance with the fourth modulation format output by the second downconverter.
In a still further embodiment, first, second and third channel selection filters are coupled to outputs of the first and second downconverters. The first channel selection filter operates to filter downconverted signals modulated in accordance with the first modulation format output by the first downconverter, the second channel selection filter operates to filter downconverted signals modulated in accordance with the second modulation format output by the first downconverter, the second channel filter also filters downconverted signals modulated in accordance with the third modulation format output by the second downconverter, and the third channel selection filter operates to filter downconverted signals modulated in accordance with the fourth modulation format output by the second downconverter.
In the above embodiments, two downconverters were coupled to outputs of the band selection filters. In a further embodiment, a third downconverter is coupled to an output of the first band selection filter, and also functions to downconvert signals from the first frequency range to the common intermediate frequency range. In this embodiment, the switching circuitry selectively couples one of either the first oscillating signal from the VCO or the second oscillating signal to inputs of the first and third downconverters and an input of the frequency doubling circuit. Whereas in the previous embodiments input signals modulated in accordance with the second modulation format were downconverted using the first downconverter, in this embodiment the third downconverter mixes the output of the first band selection filter with the first oscillating signal at the channel frequency associated with the second modulation format in order to downconvert signals modulated in accordance with the second modulation format from the first frequency range to the common intermediate frequency range. In this embodiment, one, two or three channel selection filters is/are alternatively coupled to outputs of the first, second and third downconverters, and the channel selection filter(s) operate to filter the downconverted signals modulated in accordance with the first, second, third and fourth modulation formats.